Thermo-oxidation of natural fibers and their biocomposites has attracted considerable interest from researchers due to their wide-ranging applications in the fields of textile fabrics, construction materials, insulation, and automotive components. This makes the comprehension of their thermal degradation mechanisms, especially in the presence of oxygen, a matter of critical interest. Therefore, this study aims to investigate the effect of untreated and alkali-treated Agave Americana Fibers (AAF) on the kinetics of thermo-oxidation of PHBHHx biocomposites at 30 wt.% filler content. The experimental approach consists of subjecting neat polymer and its biocomposites to different heating rates, i.e., 5, 10, and 20 °C/min, under oxygen using thermogravimetric analysis, while applying the Coats–Redfern model-fitting method. The results reveal that TGA/DTG thermograms exhibit a single-step degradation process for PHBHHx, whereas the biocomposites show multi-step degradation phenomena attributed mainly to the cellulose and PHBHHx matrix. Kinetic analysis of the samples indicates that PHBHHx degrades via a diffusion-controlled mechanism, showing the best model fitting to the D2 Model. However, both untreated and treated biocomposite samples exhibit two degradation steps related to the base components, occurring via the P2 Model. This suggests that degradation is governed by the reactions occurring at the filler–matrix interfaces. Moreover, it has been demonstrated that alkali treatment of the fibers results in an increase of activation energy and onset temperature of the biocomposites (P-AAF-B) compared to those of untreated ones (P-AAF). Indeed, at a heating rate of 5°C/min, the activation energy values increase from nearly 178 and 537 KJ/mol for P-AAF relative to phases 1 and 2 to 426 and 623 KJ/mol for P-AAF-B, respectively. Moreover, at the same heating rate, the onset temperature increases from 221.70°C for P-AAF to 236.51°C for P-AAF-B. These results show that the addition of natural fibers, especially with surface treatment, is beneficial for enhancing the material's overall thermo-oxidative stability.